Difference between revisions of "1998 AIME Problems/Problem 4"
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− | In order for a player to have an odd sum, | + | In order for a player to have an odd sum, he must have an odd number of odd tiles: that is, he can either have three odd tiles, or two even tiles and an odd tile. Thus, since there are <math>5</math> odd tiles and <math>4</math> even tiles, the only possibility is that one player gets <math>3</math> odd tiles and the other two players get <math>2</math> even tiles and <math>1</math> odd tile. We count the number of ways this can happen. (We will count assuming that it matters in what order the people pick the tiles; the final answer is the same if we assume the opposite, that order doesn't matter.) |
We have <math>3</math> choices for the player who will get three odd tiles, and <math>\dbinom{5}{3} = 10</math> choices for the tiles that he gets. The other two odd tiles can be distributed to the other two players in <math>2</math> ways, and the even tiles can be distributed between them in <math>\dbinom{4}{2} \cdot \dbinom{2}{2} = 6</math> ways. This gives us a total of <math>3\cdot 10 \cdot 2 \cdot 6 = 360</math> possibilities in which all three people get odd sums. | We have <math>3</math> choices for the player who will get three odd tiles, and <math>\dbinom{5}{3} = 10</math> choices for the tiles that he gets. The other two odd tiles can be distributed to the other two players in <math>2</math> ways, and the even tiles can be distributed between them in <math>\dbinom{4}{2} \cdot \dbinom{2}{2} = 6</math> ways. This gives us a total of <math>3\cdot 10 \cdot 2 \cdot 6 = 360</math> possibilities in which all three people get odd sums. |
Revision as of 01:26, 4 June 2014
Problem
Nine tiles are numbered respectively. Each of three players randomly selects and keeps three of the tiles, and sums those three values. The probability that all three players obtain an odd sum is
where
and
are relatively prime positive integers. Find
Solution
In order for a player to have an odd sum, he must have an odd number of odd tiles: that is, he can either have three odd tiles, or two even tiles and an odd tile. Thus, since there are odd tiles and
even tiles, the only possibility is that one player gets
odd tiles and the other two players get
even tiles and
odd tile. We count the number of ways this can happen. (We will count assuming that it matters in what order the people pick the tiles; the final answer is the same if we assume the opposite, that order doesn't matter.)
We have choices for the player who will get three odd tiles, and
choices for the tiles that he gets. The other two odd tiles can be distributed to the other two players in
ways, and the even tiles can be distributed between them in
ways. This gives us a total of
possibilities in which all three people get odd sums.
In order to calculate the probability, we need to know the total number of possible distributions for the tiles. The first player needs three tiles which we can give him in ways, and the second player needs three of the remaining six, which we can give him in
ways. Finally, the third player will simply take the remaining tiles in
way. So, there are
ways total to distribute the tiles.
Thus, the total probability is so the answer is
.
See also
1998 AIME (Problems • Answer Key • Resources) | ||
Preceded by Problem 3 |
Followed by Problem 5 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.